As the documentation mentions, the plugin has to be a separate Python package which has to be registered in the entry_points.

In my case, I have a simple script that does some pre-build operations on the documentation sources. The only way to run it automatically, for now - is to create another package that would register itself as the plugin.

How about allowing to load plugins that are just local files? E.g. - docs.custom_plugin::MyPluginClass would load MyPluginClass from the docs/custom_plugin.py. Of course, the MyPluginClass should follow the plugin API by inheriting the BasePlugin class.